Alien Invasion: Invasive Exotic Species in Minnesota

“We are in a period of the world’s history when the mingling of thousands of kinds of organisms from different parts of the world are setting up terrific dislocations in nature.” – Charles S. Elton

Along the banks of the Mississippi River in Minneapolis and St. Paul, lush green vegetation covers the slopes as they plunge down to the water. This greenery should be a welcome sight in the middle of a city. However, on closer examination, a practiced eye can see that the vegetation is dominated by one species – large bushes and small trees sporting dark green leaves. This plant is buckthorn. It is an invasive plant not native to Minnesota, and it poses a threat to the ecological balance of the region.

Common and Glossy Buckthorn are just two invasive non-native species that worry naturalists all over the state. An invasive non-native species is a variety of plant or animal somehow imported into a foreign ecological region. When imported, this species, deprived of its natural predators or competitors, thrives and outcompetes native species. Other invasive non-native species include tartarian honeysuckle, purple loosestrife, eurasian water milfoil and zebra mussels just to name a few.

Why are Exotics a Problem?

Native to central and northern Europe, buckthorn grows aggressively even in this state’s harsh climate. It is one of the first plants to bud in the spring and one of the last to drop its leaves in the fall. This plant can quickly dominate an area, shading out and eventually killing other vegetation, as can be seen along the Mississippi’s slopes. In some of these areas, native vegetation has virtually disappeared. Consequently, erosion concerns prevent the removal of the buckthorn. Buckthorn also resprouts quite readily when cut down, so stumps must be removed or carefully treated with herbicide.

Zebra mussels arrived in Minnesota by boat, in the bilgewater of international shipping vessels entering Great Lakes waterways. The tiny mussels have become a significant problem in the St. Croix River along the Minnesota/Wisconsin border near the Twin Cities. They have become so abundant that they have created dead zones where no other plants or animals can thrive.

Invasive exotics can seriously threaten biodiversity in an area. When exotic species crowd out native plants and animals, they dominate an area. This domination reduces habitat for rare species, degrades our already threatened native prairies and woodlands and eliminates cover and food sources for native fish and animals. Research also suggests that some exotic species such as honeysuckles deposit chemicals in the soil that inhibit the growth of other plants.

How Did They Get Here?

Some of these species, such as zebra mussels, purple loosestrife, and eurasian water milfoil, were inadvertently imported in the bilgewater of international commercial ships. Others, such as common and glossy buckthorn and tartarian honeysuckle, were introduced by nurseries many years ago. Many of the beautiful hedges lining Summit Avenue in St. Paul are composed of buckthorn or exotic honeysuckles. Over the years, the plants thrived Minnesota’s climate and spread. At the time they were imported, no one expected these plants to become invasive.

What is Being Done?

Many organizations across the state work to control non-native, invasive species in their specific areas of concern. The problem is too big for one single group to control. Great River Greening is one organization that works to control exotic species on the banks of the Mississippi River. There are many other organizations that do similar work in other areas.

The following small list of exotic species – many of which you might find in your own yard or park – provide a severe degree of threat to the ecosystem :

Exotic Honeysuckles

Lonicera tartarica, Lonicera morrowii, Lonicera x bella

Common and Glossy Buckthorn

Rhamnus cathartica , Rhamnus frangula (aka columnar buckthorn)

Garlic Mustard

Alliaria petiolata

Spotted Knapweed

Centaurea maculosa

White and Yellow Sweet Clover

Melilotus alba and M. officiinalis

Reed Canary Grass

Phalaris arundinacea

The following cause a moderate to severe degree of threat:

Black Locust

Robinia pseudoacacia

Siberian Elm

Ulmus pumila

What You Can Do

* Clean off boats to prevent spreading aquatic exotics.

* Volunteer to help with weed-pulling efforts.

* Remove these plants from your yard. Contact local park or DNR offices for help.

* When landscaping, choose native species. There are a wide variety of attractive native plants to choose from.

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